Argentina has the most extensive and varied trails included with Iguazú Falls park entrance and if only seeing one side, Argentina is the way to go. Completists wanting to also see it from Brazil have a logistics challenge unless multiple days are available.
My original plan was to be based in Puerto Iguazú, Argentina, the night before, see the falls, then cross over to Brazil, make use of the storage lockers at the park, and then continue to my flight at Brazil’s Foz do Iguaçu Airport (IGR), which is only about 3 km from the park.
Plan B had to be activated due to delays in Paraguay and I ended up based in Brazil. The benefit was a delightful guest house, Pousada Sonho Meu, near the bus terminal with a proprietor willing to allow me to store my bag and come back for a pre-flight shower. Downside is that the buses to Puerto Iguazú do not start running until 08:00, far to late for the plan to work.
Only taxi of the day was at 6:30 over to Puerto Iguazú, about USD 20. Brazil does not care much about stamping people in and out on that border so if going for a day trip, you can skip it, but if not returning to Brazil, best to get stamped out for future trips. Argentina does stamp everyone coming and going.
Arrived Puerto Iguazú bus terminal by 07:00. No ATMs or currency exchange nearby, the nearest ATM several blocks downhill. Argentina pesos a must for the bus to the park, ARS 10 each way, and the park itself only accepts cash in ARS, tickets now ARS 130 for most nationalities.
The first bus to the park leaves about 07:10 and arrives about 07:45, in time for the 08:00 park opening.
Be the first in line and have the park to yourself, except for guests of the Sheraton Iguazú inside the park. The first train within the park is not until 08:30 so there is a buffer for those who do not want to hike up to the trails.
Get out by 12:00 to ensure a visit to the Brazil side. 13:00 might work if everything falls in place.
Take the bus back to Puerto Iguazú, at the terminal change to a Foz do Iguaçu bus (payable in ARS or BRL) and head for the border. Those needing/wanting a stamp at the Brazilian border need to explicitly tell the driver to let them off and then the bus will speed a way, leaving you with a transfer ticket for the following bus, valid only on that company, which will be about 30 minutes later.
Back on a Foz do Iguaçu bus, ask the driver to let you off where you can connect to a Parque Nacional/Aeropueto bus bound for the Brazilian park, rather than going all the way to the city.
I needed a stamp, and then for the park-bound bus, just barely missed a bus speeding away which cost another 30 minutes, but still arrived at the park about 2:20, plenty of time for a leisurely visit!